An electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles dispersed in a dielectric solvent. An EPD typically comprises a pair of spaced-apart plate-like electrodes. At least one of the electrode plates, typically on the viewing side, is transparent. An electrophoretic fluid composed of a dielectric solvent with charged pigment particles dispersed therein is enclosed between the two electrode plates.
An electrophoretic fluid may comprise one or more types of charged particles. For a color electrophoretic display device, a fluid usually comprises at least three types of charged particles. Among them, there are white, black and non-white and non-black particles. For the non-white and non-black particles, organic pigments are often used due to their superior coloring strength.
But in practice, there are only a few types of surface modification technique that can be used to prepare organic pigment particles to allow them to be functional in an electrophoretic display. If organic pigment particles of different colors coexist in a fluid and they have the same surface chemistry, these particles may be difficult to be separated under a given driving voltage. This Is due to the fact that it is difficult to prepare these particles to have desired charge polarities and different levels of charge potential since they have the same surface chemistry.
Furthermore, if charge control agent(s) is/are used in a fluid, the different colored organic pigment particles having the same surface chemistry would compete for the same charge control agent, resulting in unstable charges during driving. All of these factors could lead to poor color performance of a display device, such as presence of color tinting and poor color contrast.